qemu-static from ubuntu/xenial contains a required fix
and now we can execute compilation for ppc64le in travis.

v2: rebase on criu-dev
v3: set QEMU_CPU=POWER8 to avoid segfaults of qemu-ppc64le-static

> > By default, both qemu and gcc5 produce code on ppc64el which will run on
> > POWER7 platform.  However, in debian gcc build instruction, gcc is explicitly
> > built to produce code for POWER8 platform, which, obviously has additional
> > instructions.

v4: use ubuntu:trusty. xenial and wily has a bug:
Processing triggers for libc-bin (2.21-0ubuntu4.1) ...
/sbin/ldconfig.real: 5: /sbin/ldconfig.real: Syntax error: ")" unexpected
/sbin/ldconfig.real: 5: /sbin/ldconfig.real: Syntax error: ")" unexpected

v5: arm, x86, aarch64 were removed by mistake
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>